Question: The scale on a map is 7cm : 6km. If two cities are 21cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 7cm on the map corresponds to an actual distance of 6km. A distance of 21cm on the map is the same as 3 $\cdot$ 7cm. The actual distance between the two cities is 3 $\cdot$ 6km, or 18km.